Optical scanners are capable of producing a representation of the image of an original by projecting an image of the original onto an optical sensor by means of an optical system. The optical sensor thereby provides a signal which represents the image of the original, or a part thereof as intensity levels.
In a practical embodiment the optical sensor is provided as a line detector. But, due to the fact that a line detector is capable of imaging only one scan line on the original at a given point in time, the original is moved past the line detector at a feed rate in order to obtain an image of the entire original, that is the original is scanned. Alternatively the line detector may be moved past the original at a given feed rate.
Various types of photosensor devices may be used as a line detector, e.g. charge coupled devices (CCD's). A CCD builds up an electrical charge in response to the exposure to light. The magnitude of the electrical charge built up is dependent on the intensity are the duration of the light exposure. The intensity of a picture element (pixel) on the original is imaged by a CCD cell. The charge built up in each CCD is sampled and discharged at sampling intervals. Typically the CCD's axe sampled at regular sampling intervals, i.e. at a constant sampling rate. Each sample represents the imaged intensity of a pixel.
The object of scanning an original is to detain a digital representation of the information comprised by the original. The digital representation it typically stored or otherwise processed on a computer that has access to some type of storage media e.g. a harddisc drive. The computer is typically involved in executing a number of different tasks that require varying computational efforts. The computer is thus exposed to a varying load. Therefore, in some situations the computer will be able to receive data at a high rate and in other situations the computer will be able to receive data at a low rate. In some situations transmission of data from the scanner to the computer may be halted for a period of time.
In order to overcome the problems associated with data transfer from the scanner to the computer due to the varying capability of the computer to receive data, a number of methods have been proposed.
A first method suggests that the scanner is provided with an internal buffer that is large enough to hold the entire image representation produced by a scan. The scanning of the original is thereby not affected by the varying capability of the computer to receive data. This method is useful for desktop scanners, but for large format scanners producing images of a size varying from 500 to 1000 megabytes it is not feasible.
A second method suggests that the scanner is operated in intervals, such that the scanner scan at a nominal feed rate while the computer is able to receive data, and when the computer is not able to receive more: data or at least not able to receive data at a maintained data rate, the scanner is halted. However, when the scanner is halted and afterwards when the scanning is resumed, mechanical vibrations caused by displacement means starting abruptly results in distortion of the digital representation of the scanned original. Therefore, this method requires that the scanner is so mechanically stable that no image distortion occurs when the scanner stops and starts. This is possible for slow speed scanners, but for fast scanners that scan at a feed rate larger than 2 inch/second this solution is not feasible.
A third method suggests that the original is initially scanned at a nominal feed rate thereby generating data transferred to the computer at a first data rate. If the computer is not capable of receiving data at this first data rate, the scanning is stopped and then restarted to scan the original at a lower feed rate, thereby generating data transferred to the computer at a second and lower data rate. This is continued until scanning of the entire original succeeds. This method is useful, but it may cause the scanning of an original to take a long time because the scanning is stopped and restarted repeatedly, if a relative low date rate is required for the transfer of data from the scanner to the computer.